It's a classic case of Catch 22: almost all airlines on short-haul routes long ago stopped serving complimentary food. Yet buying a meal at airports before flying can cost an arm and a leg.

Our study of three main food outlets at Stansted — Pret a Manger, Leon and Itsu — has found prices can leap by as much as 42 per cent compared with high street branches. Overall, prices across a basket of offerings were up by a fifth.

The single biggest increase was for a pain au chocolat at Leon — up from £2.10 to £2.99 (a 42 per cent increase) — while a cheddar and pickle baguette at Pret sold for £4.50 on the high street was £6.20 at Stansted (up 37 per cent) and their chicken caesar bacon baguette, £7.20 instead of £5.99(up 20 per cent).

Other big price hikes were at Itsu, where its Itsu Classics set, comprising sashimi, salmon, prawn sushi, wasabi and soy sauce, sells for £11.99 at Stansted, while the same product is £8.95 on the high street — up a third. Meanwhile its lemon-flavoured Zen Water was £2.49 a can, up from £1.99 at other non-airport shops.

However, fast-food chain Leon was found to mark up prices most consistently, with the average being more than a quarter on a basket of items including its pains au chocolat, butter croissants (up 39 per cent from £2 to £2.79) and signature turmeric honey, banana and bee pollen yoghurts (up 33 per cent from £2.99 to £3.99).

Rory Boland, Editor of Which? Travel said: 'The mark-ups at airport shops, cafes and restaurants can be truly shocking.

Our study of three main food outlets at Stansted — Pret a Manger, Leon and Itsu — has found prices can leap by as much as 42 per cent compared with high street branches

'Once passengers are airside they're essentially a captive audience, but it's outrageous that businesses think they can get away with such blatant price hikes.'

Rising airport and plane trolley service food costs have been pushing more and more flyers to pack picnics.

Boland said: 'Fortunately with a little forward planning, it's possible to avoid the rip-off prices and get the satisfaction of knowing you won't be handing over your hard-earned cash for an overpriced snack.

'Many foods can be taken through security, so whether you make a sandwich or buy one at a supermarket you'll likely save — just leave liquids of more than 100ml at home.'

Almost all airports have ample fresh drinking water fountains for filling water bottles — making DIY dining on planes easier than ever, while keeping costs down. Food retailers argue higher rents and labour costs in airports push up prices.

Other big price hikes were at Itsu, where its Itsu Classics set, comprising sashimi, salmon, prawn sushi, wasabi and soy sauce, sells for £11.99 at Stansted (pictured)

They also say maintenance costs are higher, and logistics and distribution more expensive. A Pret spokesman said: 'Like all businesses operating in airports, we face higher rents and running costs due to longer trading hours and night shifts.

'Our food is freshly handmade each day in our kitchens within airports, whereas some businesses manufacture their food elsewhere. We always try to absorb these costs as much as possible, but it does mean there are some price differences in our airport shops.'

Not all fast food/cafe outlets at Stansted hike prices. Prices at popular chain Joe & The Juice were identical to those in its high street branches when Escape checked.

At WH Smith a Coke (500ml) was 16 per cent more on the high street and a tea 4 per cent higher — but other prices were identical.

The Daily Mail's Holiday Guru said: 'Pack a picnic. There's no shame in taking out your food on a plane. With prices so high it's a surprise more people aren't doing it.'

Itsu and Leon were contacted but did not respond.